Many conventional low thermal expansion coefficient glasses are generally found in the “borosilicate glass family.” Borosilicate glasses have a low thermal expansion coefficient, about one-third to one half that of many boron-free silicate glasses. Typically, these borosilicate glass compositions are about 70-80 weight percent silica, 10-15 weight percent boron oxide, up to 8 weight percent sodium oxide, up to 8 weight percent potassium oxide, and minor amounts of calcium oxide (lime) and aluminum oxide. Borosilicate glasses are well known for their excellent thermal stability (low thermal expansion). This is primarily due to their relatively high silica and boron oxide content. These same glasses are, however, relatively difficult to melt (have high viscosity), for the same reason. In addition, these glasses tend to have relatively low elastic moduli (<70 GPa) and as a result are a poor choice for applications that require rigidity and high dimensional stability.
Printed circuit boards (PCB) necessarily require electrical, mechanical and thermal stability. If a glass fiber is used in a PCB as part of an insulative component, it is desirable for the glass to have a low thermal expansion coefficient (CTE), high elastic modulus and be free from hollow filaments (trapped bubbles within the fibers). In some situations, the glass fiber in combination with a polymer binder or matrix can provide an insulative material that closely matches the metal wiring and other components of the electronic device. A typical printed circuit board, for example, has a circuit pattern composed of an insulating layer and a metal, e.g., copper (Cu), gold (Au), or aluminum (Al). Metals, such as copper, have thermal expansion coefficient of about 17 ppm/° C. A glass fiber filler in combination with a plastic binder or matrix as an insulating layer can be used to more closely match the CTE of the metal. Ideally, the combination of glass fiber and plastic binder is of a design such that it reduces residual stress after the printed circuit boards are manufactured and reduces delamination of the insulating layers during use.
There are different types of glass fibers that currently find use in electronic circuit boards and specifically for IC chip carriers, such as E-glass, L-glass, and T-glass (a type of S-glass fiber). While S-glass may provide properties suitable for such applications, due to processing constraints, it is difficult to continuously process S-glass with low “hollow filament” count (the absence of a long hollow interior in the fiber, likely formed from trapped gas bubbles or seed crystals). E and L-glass fibers have on the one hand, very low hollow filament counts, but rather poor thermal expansion compatibility behavior and low elastic moduli. T-glass possesses both excellent thermal expansion and high modulus, but like S-glass, suffers from high hollow filament counts.
With continuous development in mounting technology demanding lower electric permittivity of materials, thinner substrates, and 3-dimensional packaging technology, and highly densified printed circuit board technology, improved low CTE glass fibers that have the required high elastic modulus and low hollow filament characteristics are needed that are more readily and more economically manufacturable, while still suitable for fiberization.